Maltol (2-methyl-3-hydroxy-4-pyrone) is a heterocyclic aroma chemical used extensively in flavor and fragrance compositions. It is naturally occurring in numerous plant species, especially in coniferous trees such as Larix and Abies spp.
The presence of maltol in various plant sources has been known for many years and considerable efforts have been made to develop a sensible method for its commercial recovery. The existing techniques are rather complex and the use of the resulting maltol is cost-prohibitive.
One means of purifying maltol is by co-distillation with ethylene glycol. The solubility of maltol in ethylene glycol at ambient temperature exceeds 4%. This completely prohibits economical maltol recovery from dilute mixtures, effectively eliminating virtually all natural sources. Moreover, there is a limiting practical consideration being that the crystallization of maltol from ethylene glycol at ambient temperatures is very slow. At low temperatures the viscosity of ethylene glycol also considerably hampers filtration of maltol from ethylene glycol/maltol mixtures.
Ethylene glycol derived maltol is also unsuitable for food application, since the removal of toxic ethylene glycol contaminates from maltol is rather difficult.
Maltol can be obtained in very small amounts from the destructive distillation products of wood, and by a partial synthetic process from kojic acid, which is obtained from fermentation media. However, maltol obtained from such means is quite expensive.
Maltol has also been reported to be in the bark of species of larch trees. Maltol can be present in larch bark in combined form to an extent varying from about 0.1 to about 2 percent by weight depending upon the bark layer and the season harvest. The richest supply of maltol is found in the bark roots of the larch trees although, for practical reasons, not much root bark is harvested. Large quantities of larch trees and bark containing maltol exists and are available primarily in the northwest part of the United States and southwest Canada. The bark is available at sawmills where it is stripped off of larch trees and stored in a pile to be later removed and burned for fuel or to be alternatively used in economical processes for recovering useful components.
It is also known that maltol is present in various parts of coniferous species found in rather low concentrations in the oleoresin extracted from fresh foliage of balsam fir (Abies balsamea L.)
Fleisher & Fleisher, "Water-Soluble Fractions of the Essential Oils", Perfumer and Flavorist, Vol. 16, May/Jun. 1991, pp. 37-41 gives the composition and details of recovery of compounds from fir needles (Abies balsamea L.) and a good biographical literature discussing the recovery of essential oils.
Goos and Reiter, "New Products from Wood Carbonization", Industrial and Engineering Chemistry, Vol. 38, No. 2, February 1946, pp. 132-135 discloses the isolation of maltol in small amounts by fractional distillation of soluble tar fractions.
LeBlanc and Akers, "Maltol and Ethyl Maltol from the Larch Tree to Successful Food Additive", Food Technology, April 1989, pp. 78-84 provides a survey of the historical production, properties and applications of maltol and ethyl maltol.
U.S. Pat. No. 5,221,756 to Fleisher et al. discloses that sufficiently pure maltol can be effectively recovered from oleoresin through the co-distillation with a suitable hydrocarbon and, in particular, alpha-pinene. The process of co-distillation, although, effective, requires a rather complex technological set-up, application of vacuum, high pressure steam and necessary handling of flammable liquids. Moreover, during co-distillation, maltol crystallizes directly from the gaseous phase in a microcrystalline form. Thus, maltol obtained from co-distillation processes retains substantial quantities (30 to 40%) of the hydrocarbon which complicates further purification.
On the other hand, fir balsam resin is a valuable product for the perfume industry for its fine delicate organoleptic qualities. During co-distillation this material can suffer from long exposure to high temperatures which alters the organoleptic profile of the resin and consequently reduces its value.
Despite the current state of the art there remains a continued need to provide a process for aqueous extraction of maltol from source materials containing maltol that provide commercial quantities of maltol which are in a substantially pure state. The present invention provides such a process for the recovery and purification of maltol without excessively large expenditures in equipment and raw materials.